The present invention relates to a preparation method of digital prints by which post cards are printed by a digital printer according to the data including the digital character information and the digital image information, and to an apparatus thereof.
Post cards in which characters and images are combined with each other, are conventionally known. As a method to prepare such the composite post card, other than a classic method in which characters are expressed by an ordinary printing technique using letterpress printing, or lithography, on images expressed by silver halide photography, a method in which both of the character information and the image information are made to the digital information, and printed by a digital printer, is spread in common together with a digital camera.
In the conventional printing method, when the printing is conducted on the same sized printing paper, both of the character information and the image information are processed as one digital data of 8 bits for each of, for example, R (red), G (green), B (blue), and at the time of printing, the printing is conducted by using a LUT (Look Up Table) for controlling the digital data to the exposure characteristic appropriate to the coloring of the print paper and for outputting it. However, because the color and the image density are different in the character information and the image information, when the character data and the image data are converted by the single LUT, in the print, there is a case where disharmony is generated in a character portion and an image portion. For example, specifically for a fine line, MTF is lowered, the density is decreased, or because the short time exposure characteristic and MTF (gradation) are different depending on the color, the color balance is changed, and therefore, when the print of the character image is conducted by the LUT for general image use, there is a problem that the desired color is hardly obtained. Specifically, in the character which is expressed in many cases by fine lines, the influence of the MTF is easily generated. Further, the influence of the MTF is different for each color (R, G, B). Therefore, a black fine line is reproduced to the different color from “black” due to the influence of the MTF, and the disorder of the color balance of black used as the character generates the reddish or bluish color tone which easily attracts peoples notice (conspicuous).
The above description will be detailed below.
As shown in FIG. 6, there is a color image output apparatus 1 such as color printers by which the color image is formed by coloring the color material of 3 colors of C (cyan), M (magenta), and Y (yellow) at a predetermined gradation. In this apparatus, after the input image data RGB of 3 colors is gradation-converted by an LUT 5 having one dimensional LUTs (Look Up Table) 2-4 for gradation correction, it is supplied to an exposure section 6. The exposure section 6 is structured such that laser diodes which emit the R, G, and B-colored light beams are driven corresponding to the output image data RGB of 3 color phases after the gradation correction by one dimensional LUTs 2-4, and each laser light beam L is irradiated on the color photosensitive material (paper), thereby, a latent image is formed on the color photosensitive material (paper), and by conducting a predetermined development processing on the color photosensitive material (paper) on which the latent image is formed, the color photosensitive-material (paper), on which an image of 3 color phases of CMY as a visual image is formed, can be obtained.
The color image output apparatus 1 as described above, is used, for example, as a proofer (called printing proofer) of a color printing machine. A reason in which the color image output apparatus is used as the printing proofer, is as follows: before the actual color print is prepared by a color printing machine using a rotary press, a proof print on which a color image is formed for the proof, (called color printing proof) is prepared, and because, in the printing proofer, a process of the press plate which is necessary for the color printing machine, is not necessary, a color print (a hard copy on which the color image is formed) can be prepared a plurality of times and easily, in a short period of time.
That is, by simulating the color of the color print to be prepared by a color printing machine which is to be used from now by using the trial print of the color image output apparatus 1, the color can be easily confirmed in a before process of the actual printing.
Incidentally, in this type of color image output apparatus 1, there is no possibility that the gradation correction characteristics (called also gradation characteristics) of LUTs 2-4 which are previously installed in the apparatus corresponding to any printing condition (condition of ink, sheet and printing machine itself), perfectly coincide with the printing condition (desired printing condition) of the printing machine actually to be used by the user. Therefore, when the printing proof corresponding to a desired printing condition is prepared, for optimizing the dynamic range of the density corresponding to the desired printing condition, it is necessary that the gradation characteristics of LUTs 2-4 are corrected (modified), and the LUT is optimized corresponding to the desired printing condition. In the same manner, it is necessary that the temperature change of the dynamic range is divided into several ranges, and the optimum LUT is respectively prepared for each of ranges.
Accordingly, for example, for the input image data RGB, when the target gradation (target density gradation) of each color of CMY on the film F is set to the target gradation (target density gradation) Dc0, Dm0, Dy0 as shown in FIG. 7, corresponding to the desired printing condition, it is necessary that the gradation characteristics of each of reference one dimensional LUTs 2-4 previously installed corresponding to the reference printing condition are corrected (modified) so that the input image data RGB coincides with these target gradations Dc0, Dm0, Dy0, by each of one dimensional LUTs 2-4 constituting the LUT 5 for the gradation correction.
In this case, in the gradation correction method in the conventional color image output apparatus 1, as shown in the flow chart in FIG. 8, initially, each of the image data R, G, B constituting the input image data RGB is respectively increased by a predetermined gradation, supplies to the exposure section 6 through each of reference (originally installed in the color image output apparatus 1) one dimensional LUTs 2-4, outputs the monochromatic patch of each color of CMY (step S1) onto the film F by the laser light beam L of each color of RGB, and the density Dc, Dm, Dy of each monochromatic patch is measured (step S2).
Next, the measured density Dc, Dm, Dy and the target gradations Dc0, Dm0, Dy0 are compared for each of the patch having a predetermined gradation, and the difference is outputted (step S3), and it is judged whether the difference is a value within the desired difference (step S4).
However, in the objective print and the output patch, because the spectral characteristic is different due to the difference of the coloring material, even when the density value coincides with each other, a tone of color when it is looked at, is different from each other. Accordingly, the judgement in step S4 is NO, and by correcting the corresponding relationship (conversion relationship) of each of the RGB values of the input image data RGB and each of RGB values of the output image data RGB of the one dimensional LUTs 2-4 by trial and error corresponding to the difference found in step S4, one dimensional LUTs 2-4 are corrected (step S5).
Then, by repeating processings from step S1 to step S5 until the judgement in step S4 is accomplished, one dimensional LUTs 2-4 after the gradation correction in which the measured density Dc, Dm, Dy and the target gradations Dc0, Dm0, Dy0, are values within a predetermined range, are obtained.
In the gradation correction method in the above-described conventional color image output apparatus 1, because the correction (modification) of the gradation characteristic (input output corresponding relationship) of one dimensional LUTs 2-4 is conducted by trial and error, according to the correction of one dimensional LUTs 2-4 corresponding to the above difference, one dimensional LUTs 2-4 after correction is used and the monochromatic patch is printed out, the density measurement is conducted for each print out, and because it is necessary that the operation comparing to the target gradation is repeated many times, a lot of time is necessary, and there is a problem that the high degree of skill is necessary for the determination itself of the correction amount according to the difference.
Japanese Tokkaihei No. 11-262033 teaches that, in order to solve the above problem, when the character information and the image information are composed, by composing them after conducting the predetermined processing on the character information and/or image information, the character portion and the image portion are harmonized.